Lawyers call it the "Friday factor" -- jurors' inclination to come to a verdict on Friday afternoons. It's unscientific but powerful, and it seemed to be at work on Friday, November 12. That afternoon, one particular jury -- sequestered and sworn to decide the fate of double-murder defendant Scott Peterson -- surprised the nation by announcing it had reached a verdict.

As readers very probably know, Peterson, now 32 years old, was charged with the murders of his wife, Laci, and his unborn son.

The case had been submitted to the jury nine days earlier. But that didn't mean the jurors had deliberated for nine days. Instead, a very unusual turn in the case, two of the original deliberating jurors had been unceremoniously ousted and replaced by alternates during deliberations. Under California law, that required deliberations to begin "anew" - so the replacements could fully participate in the process, and so Peterson could have the benefit of a true twelve-person jury.

Thus, if jurors followed the legal instructions properly, the deliberations that ended that Friday had begun only seven hours earlier - when the second alternate was seated. No wonder Peterson's well-known defense attorney, Mark Geragos, had headed out of town. Who would have expected this lengthy trial to come to a conclusion so quickly?

The trial had spanned five long months and included testimony from 175 prosecution witnesses. The defense, too, had put on a case - rather than resting on its attacks on the prosecution's evidence. The case was widely acknowledged by commentators to be difficult to prove - for it was based on circumstantial evidence, and some of the evidence that ought to have existed, based on the prosecutor's theory, seemed to be missing. Most conspicuously, the prosecutor claimed Scott killed Laci in their house - but there was no blood or similar evidence to bear out that contention.

Still, the jury managed to reach a quick verdict. And its verdict, as the nation now knows, was "Guilty." One might have guessed it when the jury members, entering the court room, did not meet Peterson's eye.

For Laci's killing, Scott Peterson was found guilty of murder in the first degree - meaning the jury had found not only that he killed Laci, but he did so in a premeditated and deliberate way. For Conner's killing, Peterson was found guilty of murder in the second-degree - suggesting the jury did not find evidence of premeditation and deliberateness, as it had for Laci's killing.

The result of the guilty verdicts is that Peterson may face the death penalty. The penalty phase of the trial is set to begin Nov. 22.

Shocked commentators are now asking: With little direct physical or forensic evidence tying killer to victim, how in the world did this jury ultimately determine Peterson's guilt beyond a reasonable doubt?

Many expected the victim's families would win their civil cases, based on the more forgiving "preponderance of the evidence" standard, but that the criminal case jury would hang, or even acquit, because of the bald lack of incontrovertible (or virtually incontrovertible) evidence of Peterson's guilt.

In this column, I will discuss a possible explanation for the verdict - and for its speed. It will argue that the jurors' dislike for the defendant may have provided prosecutors with a much needed shortcut in their quest for a conviction.

The Prosecution's Thin, Poorly-Presented Case

The prosecution, despite its lengthy presentation, failed to convincingly establish a definitive manner, means, or cause of Laci's tragic death. Instead, it seemed, at times, to offer the jurors "multiple choice" theories - refusing to settle on a single account of what actually occurred.

At times, the prosecution suggested that Scott killed Laci so he could live a bachelor's life, unencumbered by wife or child. The again, it suggested that Scott killed Laci so he could marry Amber Frey - whom he'd known for barely three weeks - and move in with her and her young child. It might as well have confessed the truth: It had no idea what Scott Peterson might have been thinking.

The prosecution also had to, in effect, confess ignorance as to the means and cause of Laci's death. It suggested Scott killed her in their house - but as noted above, the lack of evidence at the house sharply undermines that theory. Again, the truth is: The prosecution simply didn't know. Jurors could choose their own theory; the prosecution didn't offer them one.

Finding the Reasonable Doubt Standard Was Met: How Did the Jury Do It?

Even a casual "Law & Order" fan knows that, in any criminal trial, the burden is on the prosecution to prove the defendant's guilt beyond a reasonable doubt.

In California, "reasonable doubt" is defined as follows:

"It is not a mere possible doubt; because everything relating to human affairs is open to some possible or imaginary doubt. It is that state of the case which, after the entire

comparison and consideration of all the evidence, leaves the minds of the jurors in that condition that they cannot say they feel an abiding conviction of the truth of the charge."

How could the jurors feel an "abiding conviction" of the truth of these murder charges? Certainly, there was suspicious evidence: It would have been a marked coincidence if Laci's body just happened to wash up near where Scott was fishing the day she disappeared. But coincidences do happen.

My own strong suspicion is that the jurors' "abiding conviction" came from the infamous "Amber Frey" tapes - and it began as an abiding conviction that Scott Peterson was a loathsome human being, and morphed into an abiding conviction of his guilt.

The tapes provided no evidence at all that Scott Peterson killed his pregnant wife. They showed, instead, that he'd cheated on his wife, he'd lied to his mistress, and he wanted to keep right on lying and cheating despite his wife's disappearance. They also demonstrated that Scott had an uncanny ability to lie to even his closest family members, and exhibited a painfully callous disregard for his missing, pregnant wife. While others conducted a massive search effort and somber vigil, he tried to make sure his mistress wouldn't leave him, now that she knew the truth.

Loathsome, yes; evidence of murder, no.

Yet the effect the tapes had on the jury probably dwarfed that of any other evidence, any jury instruction, or any closing argument by counsel. Tapes don't lie - and so they provided jurors with at least some certainty in a very uncertain case.

More than that, the tapes provided the jury with a reason to hate Scott Peterson. And they likely imbued the jury a desire -- maybe subliminal, perhaps downright visceral -- to avenge the honor of a woman who tragically died ignorant to it all.

Should the Amber Frey Tapes Have Been Excluded From Evidence? Perhaps So.

Should the Amber Frey tapes have been entered into evidence? Maybe not. They contain no confession of guilt, no real clues to the crime, and no proverbial smoking gun. They were arguably of little relevance to the case. And they were unfairly prejudicial to Peterson - in that their inevitable effect was to make jurors more likely to convict, but not to demonstrate that Peterson's guilt was more likely.

That is the very epitome of the kind of unfairly prejudicial evidence that judges often exclude. Jurors ought to be convicting defendants because they believe, beyond a reasonable doubt, that they are guilty - and not for another other reason. And judges ought to keep out the type of irrelevant, or borderline relevant, evidence that practically invites jurors to convict for a reason other than the defendants' guilt.

The tapes not only helped the prosecution, they also put the defense, well, on the defensive. The tapes' proof that Scott Peterson was a Class A cad required his attorney to, in effect, use his client's bad behavior as a "theme" throughout the case. Geragos began his opening statement explaining why his client's caddish behavior did not amount to murder. And in his closing statement, he acknowledged jurors might well hate Peterson, and ended with an even stronger term than "cad" -- calling his client, quote, "a 14-carat a-hole." Geragos had no other choice. But it may have seemed to jurors that even Peterson's own lawyer was disgusted by him.

Would the outcome of this trial have been different, had the judge excluded the tapes from being admitted as evidence? I believe so. The tapes, in my opinion, allowed the prosecution a convenient shortcut on the road to guilty. Without the shortcut, the prosecution had no clear path - indeed, it did not even have any single theory, as I discussed above. And without the emphasis on Peterson's caddishness, Geragos could have spent more time on the numerous evidentiary gaps and questions, and less time admitting his own client's loathsomeness.

Certainly, every jury in a criminal trial is instructed not to be "influenced by conjecture, sympathy, passion, prejudice, public opinion or public feeling." But in reality, this instruction is little more than legal fiction. Jurors can be instructed to check their emotions at the courthouse door all day long, but until we replace human jurors with robots, emotion is going to enter the deliberation process.

The bottom lines: The tapes were moving. They probably moved the jury to anger, and even an impulse to somehow retrospectively protect an innocent woman and her innocent, unborn child. No formal instruction from a judge was going to take away that anger, or that impulse to protect.

Did this jury place emotion over substance and vote with its heart, not its head? The world may - or may not - know. Jury deliberations are the one portion of a criminal trial that occurs behind closed doors. But as in the O.J. Simpson cases, juror tell-alls may prove quite bankable here. A candid juror may well admit that the tapes were a bombshell - as I believe they had to have been.

The jury has spoken, and the message is this: We may not know how Laci was killed, when she died, or why it happened, but we know who is to blame. We find the defendant guilty beyond a visceral doubt. If that is not a verdict steeped in passion, I don't know what is.

What's Next For Scott Peterson: The Penalty Phase, and a Certain Appeal

From the prosecution's perspective, the Amber Frey tapes - relevant or not - just don't stop giving. Not only did they likely seal Peterson's doom in the guilt phase, they may actually lead to his death in the penalty face.

Now, in light of the guilty verdicts, Geragos will have to argue why the man he just called a "14-carat a-hole" should be allowed to live. And there may not be much mitigating evidence to present to support that contention - Peterson's not exactly a devoted dad who takes time off from his environmental activism to work at soup kitchens on the weekends, but snapped in a single moment of rage.

In the end, the real reason not to sentence Peterson to death - for one who believes in the death penalty, as these jurors were required to - is the same reason not to convict him in the first place: He genuinely may not be guilty; the coincidence of Laci's body ending up near where he fished may be just that: A coincidence.

Peterson's best chance at sentencing, then, may be to remind jurors of the doubts that may have worried them - but that they dismissed - at the guilt phase. Technically, guilt and innocence are not what sentencing is about: Jurors look to "aggravating" factors counseling in favor of death, and "mitigating" factors counseling against death. And possible innocence, under the law is not a mitigating factor. Still, in practice, defendants often re-raise "lingering" doubts about their guilt at the penalty phase - and Peterson may well do the same.

Should Peterson be sentenced to death, an appeal is automatic. Should he be sentenced to life without parole - the jury's only other option -- an appeal is his right. It would be shocking if he declined to file one.

An appellate court very probably wouldn't second-guess the trial court's decision to allow the Amber Frey tapes into evidence. But it might well second-guess the trial court's refusal to grant a mistrial despite recurrent problems with - and dismissals of - jurors.

If another dismissed juror comes forward - as dismissed juror Justin Falconer already has - and says he or she was leaning strongly toward acquittal, the appellate court will doubtless take a close look at the reasons why that juror, and Falconer, were dismissed. In sum, Peterson may yet win a new trial on appeal - a trial where the jury stays the same from beginning to end, or at least through its final deliberations.

Jonna M. Spilbor is a frequent guest commentator on Court-TV and other television news networks, where she has covered many of the nation's high-profile criminal trials. In the courtroom, she has handled hundreds of cases as a criminal defense attorney, and also served in the San Diego City Attorney's Office, Criminal Division, and the Office of the United States Attorney in the Drug Task Force and Appellate units. In 1998, she earned certification as a Court Appointed Special Advocate with the San Diego Juvenile Court. She is a graduate of Thomas Jefferson School of Law, where she was a member of the Law Review.